From the perspective of the compiler implementation, they're elements. But users will think of these as cases—and many diagnostics already refer to these as enum cases.
That hack resulted in already matching argument and parameter to end
up not matching in this case.
Fixes https://bugs.swift.org/browse/SR-7191
and rdar://problem/38798063
Move the existing test into Compatibility/tuple_arguments_4.swift.
Move the Swift 3 test, Compatibility/tuple_arguments.swift, into
Compatibility/tuple_arguments_4.swift.
Add a couple new tests to each file to capture what we do in each
mode.
Overall this doesn't seem like the most satisfying approach for
covering compatibility testing in our standard commit testing, but
it's all we've got at the moment.